1. Field of the Invention
The invention pertains to a bottom block system for handling a given maximum load, especially to a system for forming bottom blocks with different load capacities for cranes, especially heavy-lift cranes.
2. Description of the Related Art
Heavy-lift cranes have load capacities of at least several hundred tons. The bottom blocks used here, which have one or more load hooks, are not only rather tall, but also very heavy. In the case of a 1,000-ton crane, the bottom block with its load hook can be, for example, 4–6 meters tall and reach a total weight of approximately 35 tons. The bottom block and the load hook must be designed to handle a certain maximum load. The investment cost especially for the load hook is considerable. In most cases, however, these heavy-lift cranes lift loads which are considerably less than the allowable maximum load. In these cases, the high load capacity of the bottom block is not needed. A heavy-lift crane is often used in place of a crane with a smaller load capacity when there is a need to hoist a load while the boom is being held at a comparatively low angle and the load moment is therefore very high. The high intrinsic weight of the bottom block set up for the maximum load is therefore highly disadvantageous in this case.
To limit this disadvantage, it is known in principle that a bottom block can be formed out of two sheave blocks, for example, each of which has several sheaves. The two blocks can be hinged to each other by a triangular leveling link. Because two sheave blocks are provided, it is possible to use two lifting cables, which can lift double the load or which can be used to achieve a faster lifting speed. The load hook is attached rotatably to the leveling link in such a way that it can swing around both a horizontal and a vertical axis. A bottom block of this type can also be split; that is, the leveling link can be connected to only one of the two sheave blocks. As a result, the bottom block will have only half of the original load capacity. The disadvantage here, however, is that the load hook is much heavier than necessary, because it is still designed for the maximum load. In addition, no optimized solution is provided for managing loads which are significantly less than half the maximum load.